Whole-rock chemical composition of the pre-Neogene basement rocks and detritus garnet composition in the Okinawa-jima and neighbor islands

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  • Miyagi Naoki
    Graduate School of Education, University of the Ryukyus Present address: Elementary School Attached to the College of Education, University of the Ryukyus
  • Baba Sotaro
    Department of Natural Environment Science Education, University of the Ryukyus
  • Shinjo Ryuichi
    Department of Physics and Earth Sciences, University of the Ryukyus

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  • 沖縄島および周辺諸島に分布する先新第三系基盤岩類の全岩化学組成と砕屑性ザクロ石化学組成
  • オキナワトウ オヨビ シュウヘン ショトウ ニ ブンプ スル サキ シン ダイサンケイ キバンガンルイ ノ ゼン ガン カガク ソセイ ト サイクズセイ ザクロイシ カガク ソセイ

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Abstract

Based on whole-rock chemical compositions of sandstone and greenstone, we estimate the geologic provenance and tectonic setting of pre-Neogene basement rocks of Okinawa Island and neighboring islands. Our results suggest that sandstone clasts in the Iheya Unit were derived from a continental arc and dissected arc, whereas sandstone clasts in the Motobu Unit were formed through the mixing of various rocks derived from different provenances. Sandstone clasts in the Nago and Kayo formations were derived from similar geologic provenances (continental island arc, active continental margin, continental arc, and dissected arc), and sandstones in both formations can be divided into two groups using abundances of Nb. The chemistry of detritus garnet in sandstones from the Nago Formation indicates the existence of high-grade metamorphic rock in the provenance. The Kerama Formation has traditionally been regarded as a continuation of the Nago Formation, but the whole-rock chemical compositions of sandstones of the two formations are different. Our results, when compared with those for sandstone in the Shimanto belt, present significant differences in the contents of Sr, Cr, and Ni. These differences were presumably caused by mafic rock sources of clastic materials and by the paleoweathering conditions of the provenance.

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